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Site-to-site VPNs connect entire networks to each other -- for example, connecting a branch office network to a company headquarters network. In a site-to-site VPN, hosts do not have VPN client software; they send and receive normal TCP/IP traffic through a VPN gateway. The VPN gateway is responsible for encapsulating and encrypting outbound traffic, sending it through a VPN tunnel over the Internet, to a peer VPN gateway at the target site. Upon receipt, the peer VPN gateway strips the headers, decrypts the content, and relays the packet towards the target host inside its private network.
Remote access VPNs connect individual hosts to private networks -- for example, travelers and teleworkers who need to access their company's network securely over the Internet. In a remote access VPN, every host must have VPN client software (more on this in a minute). Whenever the host tries to send any traffic, the VPN client software encapsulates and encrypts that traffic before sending it over the Internet to the VPN gateway at the edge of the target network. Upon receipt, that VPN gateway behaves as described above for site-to-site VPNs. If the target host inside the private network returns a response, the VPN gateway performs the reverse process to send an encrypted response back to the VPN client over the Internet.
